The present invention relates to a nonstick coating for cooking utensils, especially utensils made of stainless steel, aluminum, aluminum alloy, cast iron or steel.
In this field, polytetrafluoroethylene (PTFE)-based nonstick coatings have been known for a long time. Such coatings are subject to easy scratching, to premature wear and to rapid deterioration in heat. Various means of improving the quality of these coatings, especially their abrasion resistance by the addition of fibers or oxides, and their adherence to the support by suitable anchoring sublayers, have been proposed. For example, document FR 2 768 158 described a complex multilayer coating, the sublayers of which must make the intended improvement possible; but the nonstick layer itself still consists of the upper PTFE layer, which may undergo hot decomposition at the high temperatures to which the cooking utensils, such as frying pans, are subject during cooking.
Coatings based on mixed resins or on silicone resins have also been developed, with the aim of avoiding the use of PTFE, but these techniques have encountered the same limitations.
Nonstick coatings based on quasi-crystals are also known. Unfortunately, quasi-crystals are insulating in the thermal field of cooking, which means that it is difficult to obtain cooking which is satisfactory from the consumers"" point of view (impossibility of searing meat, etc.).
Ceramics have already been used for culinary purposes, either in bulk such as glass or ceramic known under various trademarks (Arcopal(copyright), Vision(copyright), etc.) or as a coating (in particular, titanium oxide or nitride, as known from FR 2 608 592 or EP 0 489 914). In the first case, the nonstick qualities were not satisfactory, and in the second case, cracks in the coating were noted, or the coating required a PTFE coating.
The aim of the invention is to provide a new type of nonstick coating for cooking utensils, which does not have these drawbacks, which is hazard-free for the consumer and makes it possible to produce good cooking from a taste point of view.
The aim of the invention is achieved by means of a coating which is directly in contact with food, characterized in that it is chosen from the group comprising ceramo-metallic mixtures in which the ceramics are selected from titanium oxynitrides, titanium suboxides (TiOx where x is between 1.6 and 1.8), titanium subnitrides (TixN where x is between 1 and 2), spinels or mixed spinels (for example MgAl2O4, ZnAl2O4, MgCr2O4, MgFe2O4, CoAl2O4), cerium fluoride, silicon carbide (SiC), pyrolitic carbon or boron carbide (B4C), and the metal is titanium.
Titanium suboxides are known, for example, from documents U.S. Pat. No. 5,733,489, EP 0 739 325 or JP 5 009 028 which describe a method of manufacture. The ceramo-metallic mixtures are known per se and reference may be made to documents EP 0 810 982, WO 9626167, EP 0 789 007, WO 9821379 and particularly WO 9828467 which describe numerous metallo-ceramic compounds and coating methods based on these compounds. However, it is not known from these documents to use these materials to produce a nonstick coating for cooking.
The titanium-based ceramo-metallic mixtures are preferred in the invention since they give rise, possibly during the deposition phase, to titanium oxides or suboxides or nitrides or subnitrides which have the advantage of being hazard-free with regard to food and of being very stable.
Compounds of this type show good wettability, whatever the composition. The Vickers hardness is greater than 900, which guarantees freedom from scratching by cutlery (the Vickers hardness of a knife is about 500).
Preferably, the amount of titanium in the mixture is between 20% and 80%.
The thickness of the spinel phase is about 20 to 80 xcexcm.
The implementation is preferably done by dry means. A porosity having pores less than 10 xcexcm2, and a roughness characterized by undulations forming peaks separated by at least 60 xcexcm and with slopes of about 120xc2x0 to 180xc2x0 with respect to a horizontal midline, are preferred.
In the ceramo-metallic mixture, partition is necessary: the size of the titanium particles is preferably less than 60 xcexcm and that of the ceramic particles is less than 30 xcexcm. Preferably, a lenticular structure is sought.